1. Field of the Invention
The present invention relates to an objective lens element for use in an optical pickup device.
2. Description of the Background Art
As media that record a large amount of information with a high density, there are optical information storage media such as optical discs. Optical discs record information as pit-shaped patterns thereon, and are widely used for the purposes of recording digital audio files, video files, document files, and data files. Examples of functions required for performing recording, reproducing, and erasing of information on an optical disc with high reliability by using a light beam converged on a micro spot are a converging function to form a diffraction-limited micro spot, focus control (focus servo) of an optical system, tracking control, and pit signal (information signal) detection.
In recent years, due to advancement of optical system design technology and shortening of the wavelengths of semiconductor lasers which are light sources, development has progressed concerning optical discs that have a higher-density storage capacity further than ever. One approach to density increase is to increase the optical disc-side numerical aperture (NA) of a light-converging optical system which converges a light beam to form a micro spot on the optical disc. However, when the NA of the light-converging optical system is increased, an amount of a generated aberration increases with respect to a certain amount of tilt of the optical axis. In order to prevent this problem, it is necessary to decrease the thickness of a layer (hereinafter, referred to as “base material thickness”) provided on a recording surface of the optical disc. In the present specification, the “base material thickness” means a thickness from a light beam incident surface to an information recording surface of an optical disc.
For compact discs (CD) which are first generation optical discs, infrared light (a wavelength λ3: 780 to 820 nm) and an objective lens having an NA of 0.45 are used. The base material thickness of CD is 1.2 mm. For DVD which is second generation, red light (a wavelength λ2: 630 to 680 nm) and an objective lens having an NA of 0.6 are used. The base material thickness of DVD is 0.6 mm. For third generation optical discs, blue light (a wavelength λ1: 390 to 415 nm) and an objective lens having an NA of 0.85 are used. The base material thickness of third generation optical discs is 0.1 mm. As described above, as the recording density increases, the base material thickness of the optical disc decreases.
In view of economical efficiency and space occupied by an apparatus, an optical information recording/reproducing apparatus is desired which can perform recording and reproducing on optical discs having different base material thicknesses and recording densities. For this, a light-converging optical system which can converge a light beam to a diffraction limit on a recording surface of each of optical discs having different base material thicknesses, and an optical pickup device including this light-converging optical system, are necessary. In addition, when recording and reproducing are performed on an optical disc having a thick base material, it is necessary to converge a light beam on a recording surface located deeper than a beam incident surface of the optical disc, and thus the focal length has to be increased.
Prior art documents disclose configurations intended for compatible reproducing and compatible recording on an optical disc having a base material thickness of 0.6 mm and designed for the wavelength λ2 (red light) and on an optical disc having a base material thickness of 0.1 mm and designed for the wavelength λ1 (blue light).
A first prior art example is a configuration in which a wavelength-selective phase plate is combined with an objective lens. This is disclosed in Japanese Laid-Open Patent Publication No. 10-334504 and the Proceedings of ISOM2001 (Session We-C-05), P30.
As a second prior art example, a configuration in which a refraction type objective lens and a diffraction element are combined is disclosed. In Japanese Laid-Open Patent Publication No. 2004-071134, in an optical head device which performs recording or reproducing on a high-density optical disc by using an objective lens having a high NA, a sawtooth-like diffraction element is used in order to be able to also perform recording or reproducing on conventional optical discs such as DVD. The sawtooth height is set such that when blue light is used, the length of the optical path becomes 2λ, and 2nd order diffracted light is used. The sawtooth-like diffraction element emits 1st order diffracted light when red light is incident thereon. The braze direction is as in a convex lens type, and chromatic aberration compensation of the refractive lens is performed. The diffraction order when red light is used is lower than the diffraction order when blue light is used. Thus, the sawtooth-like diffraction element serves as a concave lens for red light, thereby providing an effect that the working distance can be increased.
Further, Japanese Laid-Open Patent Publication No. 2004-071134 discloses a stair-like step structure which provides an optical path difference longer than one wavelength to blue light and which provides an optical path difference shorter than one wavelength to red light. The stair-like step structure also exerts a convex lens effect on blue light and exerts a concave lens effect on red light. Thus, when blue light is used, a chromatic aberration compensation effect of the refractive lens is exerted, and when red light is used, an effect that the working distance (the interval between the objective lens surface and the surface of an optical disc) can be increased is obtained due to the concave lens effect.
As a third conventional art example, a configuration in which a relay lens is inserted between an infrared light source and an objective lens, thereby also realizing compatibility with a first generation optical disc having a base material thickness of 1.2 mm, is disclosed in Japanese Laid-Open Patent Publication No. 2004-281034.
Japanese Laid-Open Patent Publication Nos. 10-334504 and 2004-071134 merely disclose the method for compatibility with the above second generation optical discs and the above third generation optical discs. In addition, Japanese Laid-Open Patent Publication No. 2004-281034 discloses the method for compatibility with the above first generation optical discs, but requires a relay lens.
Further, it is desired that an element that realizes compatibility is integrally formed on the objective lens surface, in view of cost reduction by decrease in number of parts. However, in the prior art described above, only the exemplary configuration, in which the phase plate or the diffraction element is provided independently of the refraction type objective lens, is disclosed, and there is no description about integrally forming an element, which realizes compatibility, on the objective lens surface.
Moreover, in order to produce objective lenses at low cost and in large quantities, the material of the objective lenses is preferably resin rather than glass. In general, the material cost of resin is low, and it is also possible to mold resin at a lower temperature than to mold glass. Thus, the mold can be used long and the molding time can be shortened. Therefore, by molding resin to produce objective lenses, the manufacturing cost can be reduced. However, the refractive index of a high-NA objective lens made of resin changes due to temperature change. The refractive index change causes the refractive power of the lens surface to shift from a designed value, whereby a spherical aberration occurs. A lower-order aberration greatly deteriorates the quality of an information reproduction signal, and thus a 3rd order spherical aberration is problematic.